An example of a bolt used in a place where a male screw is subject to a repetitive stress includes, for example, a rotor spindle bolt of a gas turbine or a stud bolt of an engine crank case. Referring to FIGS. 10 and 11, description will be given below for a rotor spindle bolt of a gas turbine.
In FIG. 10, a reference numeral “100” denotes a rotor of a compressor of a gas turbine. The rotor 100 is configured by: a plurality of discs 103 that are fastened by a rotor spindle bolt (a fastening bolt for the gas turbine), that is, a bolt 101, and a nut 102; and moving blades 104 that are respectively fixed to the plurality of discs 103. The plurality of discs 103 are fastened by the plurality of bolts 101 and the plurality of nuts 102 that are disposed at approximately equal intervals in the circumferential direction of the discs 103.
In the rotor 100, thermal extension difference is generated associated with starting up and stopping of the gas turbine, and an elongation and a shrinkage are caused by projections of fluctuating loads due to thrust, a centrifugal force, or a rotation of the rotor 100 with a deformation under the weight thereof. Therefore, the fastening force of the bolt 101 and the nut 102 is varied. Thus, as shown in FIG. 11, a fluctuating stress (a tensile stress and a bending stress) is applied to an engaging portion of a male screw of the bolt 101 and a female screw of the nut 102, that is, a screw engaging portion 105. When the fluctuating stress is applied to the screw engaging portion 105, the screw engaging portion 105 tends to suffer from wear and fatigue damage. Therefore, improvement is necessary for a fatigue resistance (especially, a fretting fatigue resistance) of the screw engaging portion 105, especially, the male screw of the bolt 101.
Techniques that improve the fatigue resistance of screw fastening members (for example, a bolt and a nut, and a stud bolt) are conventionally presented (for example, Patent Document 1 and Patent Document 2). Description will be given below for a bolt that is a conventional screw fastening member. The bolt that is a conventional screw fastening member is a bolt formed by shaping a male screw that fits with a female screw, into a tapered shape in a direction along which the bolt is pulled (Patent Document 1). Another bolt is a bolt formed by molding a bolt with a round portion beneath the bolt neck thereof that has the curvature radius within a range of 1.2 to 3.0 times as large as that of a round portion beneath the bolt neck of the finished product bolt and by shaping the round portion beneath the bolt neck of the bolt by applying cold working such that the portion has a predetermined curvature radius (Patent Document 2).
However, the former (Patent Document 1) is a technique that equalizes the tensile stress applied on the male screw by shaping the male screw into the tapered shape in a direction along which the bolt is pulled, and is not a technique that increases the surface hardness of the male screw and applies a compressive residual stress to the male screw. Thus, improvement by the former of the fatigue resistance of the male screw is limited. The latter (Patent Document 2) is a technique that increases the surface hardness of the round portion beneath the bolt neck by applying cold working to the round portion beneath the bolt neck and applies a compressive residual stress to the round portion beneath the bolt neck. However, it is not a technique that increases the surface hardness of the male screw and applies a compressive residual stress to the male stress. Therefore, similar to the former, improvement by the latter of the fatigue resistance of the male screw is also limited.
Patent Document 1: Japanese Patent Application Laid-open Publication No. 1981-53651
Patent Document 2: Japanese Patent Application Laid-open Publication No. 07-180714